Variable displacement pump



March 30, 1965 M. A. DAMATO VARIABLE DISPLACEMENT PUMP 4 Sheets-Sheet 1Filed Oct. 16, 1962 0 0 w v I: V N A v I I: r 4 A mvnnon MICHAEL A. DMMTO ATTORNEY March 30, 1965 M. A. DAMATO VARIABLE DISPLACEMENT PUMP 4Sheets-Sheet 2 Filed Oct. 16, 1962 a w v mm mWJim 8 Q g QQ 3 m lmu'ronMIQHIITL A. DIMATO ILIDIIY March 30, 1965 M. A. DAMATO 3,175,510

VARIABLE DISPLACEMENT PUMP Filed on. 16, 1962 4 Sheets-Sheet s MICHAEL.DAMATO VJ fig [2 ATIQRIIZY March 30, 1965 M. A. D'AMATO I 3,175,510

VARIABLE DISPLACEMENT PUMP I Filed Oct. 16, 1962 4 Sheets-Sheet 4 mvnnonMIC HAEL A. D'AMATO ATTORNEY United States Patent 3,17 5,510 VARIABLEDISPLACEMENT PUMP Michael A. DAmato, Waukesha, Wis. (Grand Ave. andFourth St., West Des Moines, Iowa) Filed Oct. 16, 1962, Ser. No. 236,9406 Claims. (Cl. 103-162) This invention relates to variable displacementfluid pumps and motors and, more particularly, to multi-cylinder pumpsof the type in which a plurality of pistons move axially in a rotor withvariable stroke lengths, depending upon the angle of disposition of aswash plate.

In swash plate pumps, wherein the ends of cylinders in a rotor rotatesuccessively over an arcuate intake port, a blank area, an outlet port,another blank area, and then again the intake port, there arefundamental problems, one of which is to obtain and maintain goodsealing relationship between the blank surfaces at the end of the rotor,in which the cylinders are formed, and the blank areas or sealingsurfaces of the ported member or plate against which the end of therotor slides. Taking into account the requirement that the rotor must bemounted, usually on a drive shaft, on bearings in the pump casing, thetolerances, including bearing play and vibration under load necessaryfor economic manufacture of the parts generally result in relativeseparation or canting of the sliding surfaces, thereby opening the sealbetween them. For maximum efliciency, the sealing surfaces should beabsolutely fiat against one another, and should be no farther apart thanto accommodate a very thin film of oil on which the rotor end slides. Ifthe sealing surfaces separate, leakage and short-circuiting of thepumped fluid, usually oil, occurs, thereby reducing efiiciency. Cantingof the rotors end surface tends to set up localized areas of wear,either on the rotor end or on the ported member against which it slides,and before long either the rotor or the ported member must be replaced.Another important object is provision for use of a material for therotor with wear and heat expansion characteristics which are compatiblewith the axial pistons, and a bearing material for the rotating wearplate which has quite different characteristics. The object now is toprovide a wear plate mounted on the end of a pump rotor for rotationtherewith, and having significant, however limited freedom of axialmovement relative to the rotor. A further object is to provide torquetransmitting sleeves slidably fitting in the ends of the cylinders andthe ends of cylinder ports in the wear plate, and arranged to provideseals between the cylinder walls and the wear plate. By this arrangementit is intended to provide for the manufacture of the member having theinlet and outlet ports of large cross section, and also the rotor, ofdurable metal such as steel, and the rotating port plate of metal havingexcellent slide-bearing characteristics against steel, such as bronze.

Still another object relates to the formation of the ports so as toreduce cavitation to the minimum. In order that the pistons shallundergo intake strokes and suck in the pumped fluid during nearly halfof one rotation of the rotor, and then exhaust the pump fluid duringnearly all of the other half rotation, it is desirable that the inletand outlet ports occupy elongate arcuate zones, and that they be longenough so that the cylinder ends register therewith during substantiallytheir entire intake and exhaust strokes. It is also desirable tomaintain, to the extent possible, a one-to-one relationship between thecrosssectional area of the inlet and outlet ports and the inlet andoutlet openings, as well as the transistional fluid connectionstherebetween. To accomplish this within the practical limits of designand manufacture, the inlet and outlet ports assume the form of elongatearcuate slots.

l The cylinders, however, are round so as to be easily bored,

tional object is to utilize the rotating wear plate as a member forproviding transitional port shapes" anti sur faces between the roundcylinder ends and the relatively narrow elongate arcuate inlet andoutlet ports. Fur-'' thermore, because of the separable sealing sleevesand rotating wear plate, it is possible to bore and hone the cylindersby straight-through machining whereas otherwise it would be necessary towork the tools into blind inner ends of the cylinders.

Still another object is to provide, at the inner end of a rotarymulti-cylinder pump, fixed and rotating wear plates having sealing landsand bleeder grooves for bleeding a predetermined minor amount of leakageto the bearings at the inner end of the rotor shaft and to thelow-pressure side of the pump. Another object is to provide a taperedroller bearing for the outer end of the pump shaft, and to pre-load thepump shaft by light spring pressure on its inner end, thereby holdingthe tapered bearing members firmly against one another when the pump isidling and therefore not producing the reactive forces which prevent thetapered bearing members from chattering when the pump is under load.

These and other objects will be apparent from the followingspecification and drawings, in which:

FIG. 1 is an isometric view of the pump, partly broken away;

FIG. 2 is a longitudinal cross section through the pump, the sectionbeing somewhat sector-shape through the rotor in order to show theextreme positions of two of the seven pistons which are not exactlydiametrically opposed;

FIG. 3 is a transverse cross section along the line 3-3 of FIG. 2,looking in the direction of the arrows;

FIG. 4 is a transverse cross section along the line 4-.4 of FIG. 2,looking in the direction of the arrows;

FIG. 5 is a transverse cross section along the line 5-5 of FIG. 4,looking in the direction of the arrows;

FIG. 6 is an enlarged fragmentary cross section, taken longitudinallythrough the inlet port, illustrating the sealing action of the parts;

FIG. 7 is a fragmentary plan view showing a cylinder port in therotating wear plate; and,

FIGS. 8 and 9 are fragmentary cross sections along the lines 88 and 99of FIG. 7, showing the shape of the cylinder port.

Referring now to the drawings, in which like reference numerals denotesimilar elements, variable capacity pump 10 is enclosed within a casing12 formed of two separable parts 14 and 16, respectively, connected by ajoint 18 and held together by bolts 19. Casing i2 is generallycylindical and centrally mounted therein is a drive shaft 28 having anoutwardly projecting free end 21 arranged to be connected to a suitabletorque source. The inner end of the drive shaft is rotatably supportedby a roller hearing assembly 22 engaging in an annular recess 23 incasing part 16, the inner end of drive shaft 20 also engaging against anend thrust ball bearing assembly 24. A compression spring 26 engagingbetween the inner end of a bearing recess 27 and ball bearing race 28provides an axially outward thrust against shaft 20 so as to hold itfirmly against tapered roller bearings 29, thereby preventing chatteringof the shaft and bearings when the pump is idling. A conventional seal30 is provided for preventing leakage of the pump fluid outwardly frompump chamber 31. As seen best in F168. 1 and 5, casing part 16 isprovided with an inlet 32 on one side and a diametrically opposed outlet34.

Keyed, as at 36, to shaft 20; is a cylinder barrel 38 having seven axialcylinders 4% equally spaced therearound and extending from end to endtherethrough. Sliding in outer end portions of cylinders 46 are hollowpistons 42 which are reciprocated, when shaft 29 is rotated, by a swashplate 44. As shown in FIGS. 1 and 2, plate 44 is adjustably supported bystub shafts 45 which project outwardly through bearings as .on oppositesides of easing part 14. At 90 from stub shafts 46 on swash plate 44 area pair of cars 49 and at, car, 49 being engaged by plunger 50 which isslidable in a bore 51 adjacent one side of the casing, and which may beaxially adjusted by a lead screw 52 turned by a hand wheel 54. On theopposite side of the casing, a back-up piston 56 is axially slidable ina bore 58, the back-up piston 56 engagaing against a back-up adjustmentrod 69. which is axially adjustable by a screw assembly 52'. Bore 58 isconnected by a duct (not shown) to the high-pressure tional area oftransition 82 and inlet port 84 be similar,

' or nearly similar to the cross sectional area of inlet 32.

side of the pump so that, when the pump is under load,

back-up piston 56 presses firmly against ear 61, thereby exerting aforce tending to hold the opposite car 49 tightly against plunger 5e.

Swash plate 44 is faced with a reaction plate 64 engaging in an annularrecess 66 on its inner face, the reaction plate 64 being slidablyengaged by piston shoes 68 whose flat surfaces 70 slide thereagainst.Piston shoes 68 have semi-cylindrical convex surfaces 72 slidablyengaging, like knuckles, against semi-cylindrical concave surfaces 74 onthe outer ends of pistons 42. It is noteworthy that piston shoes 68 arehollow, as at '76, and communicate with the hollow interior of pistons42 so as to achieve a hydrulic near-balance between the outer ends ofthe piston shoes 68 and reaction plate 64,

the piston shoes thereby engaging against the reaction plate with butslightly more than'enough pressure to effect a seal therebetween. Pistonshoes68 are loosely connected to the outer ends of pistons 42 by cotterpins 78 so that a free swiveling motion can occur between the pistonshoes and the pistons. Compression springs 80 within hollow pistons 42provide the outward bias for the pistons. I

When barrel 38 is turned by shaft 20, those of the pistons 42 which passaround the side of the pump in which the inlet 32 is disposed moveoutwardly against the surface of reaction plate 64 on swash plate 44,thereby drawing pump fluid into cylinders 40, and those of 'the pistonswhich are then passing around the side of the pump in which outlet 34 isdisposed are cainmed inwardly, thereby expelling the pump fluid throughthe outlet. The volume of fluid drawn into and expelled from the pump,with each roation of shaft 2%, depends upon'the angle at which the swashplate 44 is disposed, it being obvious that when the surface of reactionplate 64 is normal to the axes of cylinders 4h, no pumping actionoccurs, and when swash plate '44 is tipped to an extreme position as inFIGS. 1 and 2, maximum delivery occurs. Since the pump fluid is drawninwardly and expelled via openings through surfaces at the inner ends ofcylinders 40, and since these surfaces must slide against one another toachieve the necessary valving and porting, it is vital to avoidsubstantially leakage, since leakage, or poor sealing, maybe due tonecessary tolerances entailed in eco-' nomic manufacture, or it mayresult from wear of the parts sliding against one another, or from axialmovement of the rotor, or shaft vibration, it is evident that basicproblems arise in the design of these parts which can be inexpensivelymachined but which nevertheless will maintain effective sealing againstexcessive leakage during the life of the pump. It is with these featuresthat the subject invention is primarily concerned.

Referring particularly to the details illustrated in FIGS. 3 to 6,inclusive, inlet 32, which is circular at its outer end, passes inwardlythrough a transition 32 to an arcuate inlet port 84. It is desirablethat the cross sec- On the opposite side of the pump is a similar outletport 86, which likewiseconnects with outlet 34 via a transition 88.Between the ends of the arcuate inlet and outlet ports 84 and 86 areblank areas 90, 92, the center of the area 99 representing the deadcenter inward posit on of the pistons and the center of the area 92representing the dead center outward position of the pistons. Fittingwithin a counterbore 94 in part 16 of the casing, and maintained againstrotation by a pin 96 is a fixed wear plate 98, preferably of steel andhaving therethrough arcuate inlet and outlet slots 1%, 102respectwelyregistering over inlet and outlet ports 84 and 86;. The sidewe of fixed wear plate 98 disposed towards the cylinders is fiat planar,whereas the side 106 disposed towards the bottom of counterbore $4 haslands and grooves for purposes described below.

Mounted upon and rotating with the inner end of barrel 3% is a rotatingwear plate 168, preferably of bronze or the like material having goodbearing properties for sliding against steel, and having cylinder ports110 registering with the inner ends'of cylinders 40. The ends 112 of{cylinder ports 11th which are disposed towards the cylinders 40 arecircular, whereas the ends 114 disposed towards fixed Wear plate 918 arearcuate,the' cylindrical and arcuate ends being connected bytransitional bevels 116, 117. The arcuate ends 114 of the cylinder portsregister with the arcuate inlet and outlet slots 100, 192 iii the fixedwear plate but are much shorter, being but slightly less in angularextent than the blank areas between the ends of the inlet and outletslots 11%, 102 through the fixed wear plate 93.

As shown in large detail in FIG. 6, rotating wear Plate 108 iscounterbored at the circular end 112 of each cylinder port 110 toprovide a seat 122. The inner ends of cylinders 40 are alsocounterbo'red to provide enlarge ments .124, each terminating in ashoulder w126, and a sleeve 1 28 slidably engages in both seat 122 andenlargement 124 and bridges the butt joint 129' between the. inner endof barrel 33 and the adjacent side of wear plate 108. The outer side ofeach sleeve 128 is turned down, near its outer end, to provide a recess130, and engaging over the outer end of each sleeve is a flat snapWasher 132. The inner ends 134 of piston springs 80 abut against snapwashers 132, and O-rings 136 are accommodated in recesses 130 toestablish al fiuid seal'between theouter sides of sleeves 128 and. theinner sides of cylinders 40. It should be noted that the maximumdiameters of sleeves 128 are somewhat greater than the ends of thepistons 42 which. they face so that a differential sealing pressure isexerted against the outer ends ofthe sleeves. I

Sleeves 128 serve several basic functions, the first being to transmittorque from barrel 38 to the rotating wear plate 108. By forming therotating wear plate androtor separately, it is possible to utilizedifferent metals, such as bronze for the rotating wear plate andsteelfor sleeves 1 2 8, barrel 38 and fixed wear plate 98. This alsomakes it possible to replace, if necessary, a single wearing member,

rather than one of. the larger components, such as the barrel, afterextensive use of the pump. Also, by forming the rotating wearplate 108and barrel 38 separately, it is possible to create the desired complexshape for ports 1 10, and it is also possible to hone cylinders 40and'enlargements 124 by straight through operations, rather than byWorking against blind inner ends of the cylinders. Most importantly,however, the slidability of sleeves 128 in seat 122 and enlargements 124permit a slight freedom of motion between rotating wear plate 108 andbarrel 38 so that the sealing surfaces on the inner side of the rotatingWear plate can be maintained in flat sliding engagement against the flatplanar surface 104 of fixed wear plate 98, even though the barrel 38,which is keyed to shaft 20, may

undergo slight axial or tipping movements as a result of shaftdeflection or vibration resulting from the bearingsr Rotating wear plate108 has a slight but nevertheless significant freedom of universalmovement with respect to shaft 20, resulting from the minute playrequired for the sliding fit of sleeves 128 in enlargements 124 andseats 122.

Referring now to the lower half of FIG. 3 and the upper half of FIG. 4,it will be observed that whereas the ends of ports 110 which facetowards cylinders 40 are circular, the ends which face towards the fiatside 104 of fixed wear plate 98 are arcuate and are chamfered at theirends to provide bevels 1:17 which form smooth transitions, in theannular direction of the port array, between the circular side and thearcuate side of the ports. As noted hereinbefore, the cylinder sides ofports 110 have beveled surfaces 116 which provide smooth transitions inthe radial directions of the ports between their circular and arcuatesides. Surrounding and spaced radially outward from the inner sides ofports 110 is an annular groove 140 communicating with the periphery ofrotating wear plate 108 via radial channels 142 for bleeding leakage tothe portion 144 of the chamber in which barrel 38 is mounted, thechamber being bled to the low pressure side or intake port 34 via a duct(not shown). Surrounding the inner arcuate sides of ports 110 are flatsealing surfaces 145 which slide against the flat side 104 of fixed wearplate 98.

Referring to the lower half of FIG. 4, the land and groove side 106 offixed wear plate 98, which engages against the flat plane surfaces ofcounterbore 94 (FIG. 5) is formed with fiat segmental lands 146 and 148which surround inlet and outlet slots 100, 102, respectively, and shortsegmental lands 150, the lands 146, 148 and 150 providing fiat sealingsurfaces. An inner annular groove 152 communicates with the recess 23 inwhich bearing 22 is housed, the inner annular groove communicatingthrough radial grooves 156 with an outer annular groove 158 whichsurrounds lands 146, 148 and 150. A flat annular bearing land .160surrounds outer annular groove 158, the latter being vented to the pumpchamber space 144 via bleeder notches 16-2 formed in the outer side andperiphery of counterbore 94. The bearings at the inner end of shaft 20are thus lubricated by leakage fluid which eventually bleeds to the lowpressure side of the pump.

A vitally important concept of the pump as a whole is the axial balanceof forces while providing oil passageways to and from the cylinders thesize of which are in excess of that which could be obtained in theconventional or common design and still maintain a hydraulicallybalanced or pressure loaded seal between the rotating and stationaryplates at the port section of the pump. The above concept also providestwo other very desirable auxiliary features: (1) the opportunity to usea ferrous metal in the cylinder rotor and a bearing metal in therotating wear plate; and (2) the use of a tapered roller bearing forradial loading and axial orientation. The internal radial loading ofthis bearing is a result of the combined radial reaction of the pistonsproduced by the cam plate and the magnitude of this radial force is afunction of the pressure being pumped, Because the permissable bearingradial load is proportional to the thrust load and the thrust load isalso a function of the pressure being pumped, there results an idealsituation. The thrust or force against this hearing, neglecting thatdesigned into it with the spring 26 for idling, is caused by thecylinder seal 128 being of greater cross sectional area than that ofpiston 42. This differential area multiplied by the fluid pressureequals this thrust force.

The common design for this type of pump is to float the cylinder rotoron a shaft with axial freedom. The pressure of oil behind a piston thenforces the piston to the cam plate on one end and by reducing theopening from the piston chamber to the outlet port on the other end ofthe cylinder rotor is hydraulically forced to the port end of the pump.The size of the passageways leading to and from the piston chambercontacting the port openings are very critical to pump operation. Ifthey are larger in cross sectional area than the pistons are, then theintegral type cylinder rotor and port plate would be forced forwardbreaking the seal between these faces and the pump becomes inoperative.If they are small to maintain a good seal, the oil is choked fromentering and leaving the piston, which limits the amount of oil the pumpcan pump using atmospheric pressure to fill the piston chambers.

The subject pump cylinder rotor is rigid with the drive shaft. By usingrotating wear plate 108 which is hydraulically sealed to a mechanicallyoriented, not rigid with the cylinder rotor by cylinder seals @128, thedisadvantages listed above for the conventional design are overcome. Oilpassageways can be increased because the sealing forces between the portplate 98 and valve plate 108 is increased by the increase in size of thecylinder seal over that of the piston 42. The cylinder rotor now isforced via the shaft into the tapered bearing by this differentialforce.

Another feature of fixed wear plate 98 is an arrangement which enhancesgreately the quietness in the operation of the pump. To a large extent,noise in the pump is caused by the time rate of pressure change.Referring particularly to FIGS. 3, 4 and 5, it will be noted that theland space between the ends of arcuate outlet slots 100, 102 isconsiderably greater than the blank areas 90, 92 between the ends of theinlet and outlet ports 84, 86. Holes 103a and 102a are drilled throughfixed wear plate 98 so as to locate the ends of slots 100, 102 withaccuracy far beyond that obtainable with ondinary casting practice (alsopermitting the casting of the plates :for both left and right-hand drivepumps).

When the arcuate end 114 of a cylinder port 110 is midway between theends of slots 100, 102, and assuming swash plate 44 to be tilted, then,if the lands between the ends of slots 100, 192 were the same as theblank areas 90, 92, the cylinder port pressure would go from high to lowpressure almost instantly. By using a more extensive land area betweenthe ends of the inlet and outlet slots 166, 102, about 15 of rotation isrequired for a cylinder port 110 to go from the end of slot 102 to thestart of slot 100. Small holes 100b, 102b, whose opposite ends 1021) aredenoted in dotted lines, are drilled through the fixed wear plate whichwill permit enough transfer of oil to match the cylinder pro-compressionon one side and the decompression onthe other with that of the pressuresexisting within ports 100, 102, with resultant quiet operation of thepump.

Pumps constructed in accordance with the foregoing specification havehigh volumetric efficiencies high in the s while pumping pressures inexcess of 3000 psi. The unique porting design permits the minimum ofinternal oil velocity during maximum speeds, reducing inefliciency andexcessive wear due to cavitation. Due to the extreme simplicity of theunit, the working parts are held to a minimum and those componentssubject to wear are self compensating to assure long, trouble-freeperformance under the most sever operating conditions.

The invention is not limited to the details illustrated and describedherein, but is intended to cover all substitutions, modifications andequivalents within the scope of the following claims.

I claim:

1. In a fluid energy translating device, a casing defiining a chamberhaving inner and outer ends, a port member affixed at the inner end ofsaid chamber and having fiat planar surface means facing the outer endthereof with inlet and outlet ports angularly spaced therein along arcsof a circle, a drive shaft extending fromend to end through saidchamber, bearing means rotatably supporting said drive shaft in saidcasing, a swash plate mounted in said chamber and spaced towards theouter end thereof from said port member, a cylinder barrel affixed onsaid shaft for movement therewith and having inner and outer endsrespectively facing the port member and the swash plate, a plurality ofcylinders angularly spaced about the rotative axis of the barrel andhaving inner and outer end portionsrespectively disposed towards theport member and swash plate, pistons reciprocating in the outer endportions of said cylinder and having force. transmitting means thereonengaging said swash plate for camming said pistons in one directiontowards the inner end portions of said cylinders during one phase ofrotation of said shaft, return stroke means for forcing said pistons inthe opposite direction during another phase of rotation of said shaft, arotating ,wear plate having an outer surface adjacent the inner end ofsaid barrel and an inner surface slidingly engaging the flat planarsurface means of said port member, said rotating wear plate having aplurality of passages therethrough having circular outer ends and innerends respectively registering with said cylinders and said ports,cylindrical seats in the outer surface of said wear plate respectivelysurrounding the outer ends of said passages, said barrel having rightcylindrical enlargements of said cylindeis at the inner end portionsthereof, cylindrical sleeves having circular inner end and outer endportions slidably engaging respectively in said cylindrical seats andenlargements and constituting combined fluid transmitting and fluid sealmeans between said barrel and said rotating wear plate, the innerdiameters of the sleeves being substantially equal to the diameters ofthe outer ends or" said passages.

2. The combination claimed in claim 1, a terminal part of the outer sideof the outer end portion'o-f each of said sleeves being of reduceddiameter with respect (to the diameter of the cylinder enlargement inwhich the remainder of said outer end portion slidably engages anddefining, between the outer side out the sleeve and the wall of theenlargement a cylindrical pocket, and an O ring seal in said pocket,said seal surrounding said reduced diameter part of the sleeve andengaging the wall of the enlargement.

3. The combination claimed in claim 1, the combined axial lengths ofeach of said seats and said enlargements being greater than the axiallengths of said sleeves whereby the outer ends of said sleeves arespaced from thecnds of said enlargements, said return stroke meansincluding expansion springs operatiyely engaging between said pistonsand the outer ends of said sleeves. 1

4. The combination claimed in claim 1, said bearing means comprising aradial thrust bearing supported in said casing at the inner end of thechamber, atapered roller bearing supported in said casing at the outerend of said chamber, said tapered roller bearing having bearing I pumpidles.

5. The combination claimed in claim 1, the inner ends of said passagesbeing elongate along arcs of the same circle as 'the inlet and outletports and of substantially equal radial extent as the inlet and outletports, the inner ends of said passages bcingsubstantially equal incross-sectional area to the outer ends thereof and said passages beingof substantially the same cross-sectional area from end to end.

6. The combination claimed in claim 5, said inner ends of said passageshaving arcuate lengths greater than the diameter of the ou-terends andor less radial .Width than said inner ends, there being bevelsconstituting transitional, sunface between the inner and outer ends. ofthe passages, thereby providing gradual emergence from the shape or" thepassage inner ends to the passage outer ends, whereby to reducecavitation of liquid flowing at high velocity therethrough.

References Qited by the Examiner UNITED STATES PATENTS 2,365,067 12/44Gauld, l03162 2,619,041 11/52 Born l03--162 2,646,754 7/53 Overbeke103161 2,661,695 12/53 Ferris 103-162 XR 2,844,104 7/58 Wennberg 1031622,963,983 12/60 Wiggerman 103-162 2,972,962 2/61 Douglas 1031622,992,619 7/62 Nilges -a l03162 3,124,079 3/64 Boyer 103162 FOREIGNPATENTS 1,233,394 5/60 France.

504,144 7/30 Germany. 1,020,525 12/ 57 Germany.

822,014 10/59 Great Britain.

822,692 10/59 Great Britain.

LAURENCE V. EFNER, PrimaryExaminen JOSEPH, H. BRANSON, 111., Examiner

1. IN A FLUID ENERGY TRANSLATING DEVICE, A CASING DEFINING A CHAMBERHAVING INNER AND OUTER ENDS, A PORT MEMBER AFFIXED AT THE INNER END OFSAID CHAMBER AND HAVING FLAT PLANAR SURFACE MEANS FACING THE OUTER ENDTHEREOF WITH INLET AND OUTLET PORTS ANGULARLY SPACED THEREIN ALONG ARCSOF A CIRCLE, A DRIVE SHAFT EXTENDING FROM END TO END THROUGH SAIDCHAMBER, BEARING MEANS ROTATABLY SUPPORTING SAID DRIVE SHAFT IN SAIDCASING, A SWASH PLATE MOUNTED IN SAID CHAMBER AND SPACED TOWARDS THEOUTER END THEREOF FROM SAID PORT MEMBER, A CYLINDER BARREL AFFIXED ONSAID SHAFT FOR MOVEMENT THEREWITH AND HAVING INNER AND OUTER ENDSRESPECTIVELY FACING THE PORT MEMBER AND THE SWASH PLATE, A PLURALITY OFCYLINDERS ANGULARLY SPACED ABOUT THE ROTATIVE AXIS OF THE BARREL ANDHAVING INNER AND OUTER END PORTIONS RESPECTIVELY DISPOSED TOWARDS THEPORT MEMBER AND SWASH PLATE; PISTONS RECIPROCATING IN THE OUTER ENDPORTIONS OF SAID CYLINDER AND HAVING FORCE TRANSMITTING MEANS THEREONENGAGING SAID SWASH PLATE FOR CAMMING SAID PISTONS IN ONE DIRECTIONTOWARDS THE INNER END PORTIONS OF SAID CYLINDERS DURING ONE PHASE OFROTATION OF SAID SHAFT, RETURN STROKE MEANS FOR FORCING SAID PISTONS INTHE OPPOSITE DIRECTION DURING ANOTHER PHASE OF ROTATION OF SAID SHAFT, AROTATING WEAR PLATE HAVING AN OUTER SURFACE ADJACENT THE INNER END OFSAID BARREL AND AN INNER SURFACE SLIDINGLY ENGAGING THE FLAT PLANARSURFACE MEANS OF SAID PORT MEMBER, SAID ROTATINGH WEAR PLATE HAVING APLURALITY OF PASSAGES THERETHROUGH HAVING CIRCULAR OUTER ENDS AND INNERENDS RESPECTIVELY REGISTERING WITH SAID CYLINDERS AND SAID PORTS,CYLINDRICAL SEATS IN THE OUTER SURFACE OF SAID WEAR PLATE RESPECTIVELYSURROUNDING THE OUTER ENDS OF SAID PASSAGES, SAID BARREL HAVING RIGHTCYLINDRICAL ENLARGEMENTS OF SAID CYLINDERS AT THE INNER END PORTIONSTHEREOF, CYLINDRICAL SLEEVES HAVING CIRCULAR INNER END AND OUTER ENDPORTIONS SLIDABLY ENGAGING RESPECTIVELY IN SAID CYLINDRICAL SEATS ANDENLARGEMENTS AND CONSTITUTING COMBINED FLUID TRANSMITTING AND FLUIDSEALMEANS BETWEEN SAID BARREL AND SAID ROTATING WEAR PLATE, THE INNERDIAMETERS OF THE SLEEVES BEING SUBSTANTIALLY EQUAL TO THE DIAMETERS OFTHE OUTER ENDS OF SAID PASSAGES.